Positional Accuracy of TIGER/Line® Data
The TIGER® data base was created originally to support the 1990 Census and has been updated periodically and used to support the ongoing data collection and publication work of the Census Bureau. The TIGER/Line files are the extract of the TIGER data base that is made available to the public.
During the initial creation of the Census TIGER data base in the late 1980s the principal sources of information used were the U.S. Geological Survey (USGS) 1:100,000-scale Digital Line Graphs (DLG), USGS 1:24,000-scale quadrangles, the U.S. Census Bureau's 1980 geographic base files (GBF/DIME-Files), and a variety of miscellaneous maps and aerial photographs. The DLG coverage was extensive, albeit of variable currency, and comprised most of the rural, small city, and suburban area of the TIGER data base. According to the USGS, the positional accuracy of features the DLG files is within about 200 feet of their correct position on the earth. The GBF/DIME-File coverage areas were updated through 1987 with the manual translation of features from the most recent aerial photography available to the U.S. Census Bureau.
In order to update the geographic data base from which to extract the TIGER/Line files, the Census Bureau uses various internal and external procedures to add new features (roads, boundaries, etc.) Many updates come from the geographic staff in each of the Census Bureau's 12 regional offices who work with local sources to identify new features. In recent years the Census Bureau has obtained digital files from the private sector and local officials. In the years preceding 2000 many updates came from map annotations made by enumerators preparing for the Census as they attempted to locate living quarters by traversing every street feature in their assignment area. Due to resource constraints the enumerator updates were digitized directly into the Census TIGER data base without geodetic controls or the use of aerial photography to confirm the features' locational accuracy. The Census Bureau also made other corrections and updates to the Census TIGER data base supplied by local participants in various U.S. Census Bureau programs. These local updates originated from map reviews by local government officials or their liaisons and local participants in Census Bureau programs. Maps were sent to participants for use in various census programs, and some maps were returned with update annotations and corrections. The Census Bureau generally added the updates without extensive checks. Changes made by local officials do not have geodetic control.
While it has made a reasonable and systematic attempt to gather the most recent information available about the features this file portrays, the Census Bureau cautions users that the files are no more complete than the source documents used in their compilation, the vintage of those source documents, and the translation of the information on those source documents. The Census Bureau is currently in the early stages of developing a system to significantly improve the coordinate accuracy of features in the TIGER data base and to devise a more effective approach to updating features.
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